来自文档

安全检查一 指定的初始化式必须确保所有属性 类引入的初始化 超类初始值设定项。

我们为什么需要这样的安全检查?

为了回答这个问题，让我们在swift中经历初始化过程。

Two-Phase Initialization Class initialization in Swift is a two-phase process. In the first phase, each stored property is assigned an initial value by the class that introduced it. Once the initial state for every stored property has been determined, the second phase begins, and each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use. The use of a two-phase initialization process makes initialization safe, while still giving complete flexibility to each class in a class hierarchy. Two-phase initialization prevents property values from being accessed before they are initialized, and prevents property values from being set to a different value by another initializer unexpectedly.

为了确保两步初始化过程按照上面定义的那样完成，有四个安全检查，其中一个是，

安全检查一 指定的初始化式必须确保所有属性 类引入的初始化 超类初始值设定项。

现在，两相初始化从来不讲顺序，但是这个安全检查，引入了super。Init是有序的，在初始化所有属性之后。

安全检查1可能看起来无关紧要， 两阶段初始化可以防止属性值在初始化之前被访问，而不需要进行此安全检查1。

就像这个样本

class Shape {
    var name: String
    var sides : Int
    init(sides:Int, named: String) {
        self.sides = sides
        self.name = named
    }
}

class Triangle: Shape {
    var hypotenuse: Int
    init(hypotenuse:Int) {
        super.init(sides: 3, named: "Triangle") 
        self.hypotenuse = hypotenuse
    }
}

三角形。Init在使用之前已经初始化了所有属性。所以安全检查1似乎无关紧要，

但可能还有另一种情况，有点复杂，

class Shape {
    var name: String
    var sides : Int
    init(sides:Int, named: String) {
        self.sides = sides
        self.name = named
        printShapeDescription()
    }
    func printShapeDescription() {
        print("Shape Name :\(self.name)")
        print("Sides :\(self.sides)")
    }
}

class Triangle: Shape {
    var hypotenuse: Int
    init(hypotenuse:Int) {
        self.hypotenuse = hypotenuse
        super.init(sides: 3, named: "Triangle")
    }

    override func printShapeDescription() {
        super.printShapeDescription()
        print("Hypotenuse :\(self.hypotenuse)")
    }
}

let triangle = Triangle(hypotenuse: 12)

输出:

Shape Name :Triangle
Sides :3
Hypotenuse :12

Here if we had called the super.init before setting the hypotenuse, the super.init call would then have called the printShapeDescription() and since that has been overridden it would first fallback to Triangle class implementation of printShapeDescription(). The printShapeDescription() of Triangle class access the hypotenuse a non optional property that still has not been initialised. And this is not allowed as Two-phase initialization prevents property values from being accessed before they are initialized

因此，确保两阶段初始化是按照定义完成的，需要有一个特定的顺序调用super。init，也就是在初始化self类引入的所有属性之后，因此我们需要一个安全检查1

Swift在初始化器中有一个非常清晰、特定的操作序列。让我们从一些基本的例子开始，一直到一般的情况。

让我们以一个对象a为例，我们将这样定义它。

class A {
    var x: Int
    init(x: Int) {
        self.x = x
    }
}

注意，A没有超类，所以它不能调用super.init()函数，因为它不存在。

好，现在我们用一个新类B来子类A。

class B: A {
    var y: Int
    init(x: Int, y: Int) {
        self.y = y
        super.init(x: x)
    }
}

这与Objective-C不同，Objective-C通常首先调用[super init]。但在Swift中并非如此。在执行其他操作之前，包括调用方法(包括父类的初始化式)，您有责任确保实例变量处于一致的状态。

Swift不允许你在不初始化属性的情况下初始化超类，与Obj c相反，所以你必须在调用"super.init"之前初始化所有属性。

请登录http://blog.scottlogic.com/2014/11/20/swift-initialisation.html。 它很好地解释了你的问题。

来自文档

安全检查一 指定的初始化式必须确保所有属性 类引入的初始化 超类初始值设定项。

我们为什么需要这样的安全检查?

为了回答这个问题，让我们在swift中经历初始化过程。

Two-Phase Initialization Class initialization in Swift is a two-phase process. In the first phase, each stored property is assigned an initial value by the class that introduced it. Once the initial state for every stored property has been determined, the second phase begins, and each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use. The use of a two-phase initialization process makes initialization safe, while still giving complete flexibility to each class in a class hierarchy. Two-phase initialization prevents property values from being accessed before they are initialized, and prevents property values from being set to a different value by another initializer unexpectedly.

为了确保两步初始化过程按照上面定义的那样完成，有四个安全检查，其中一个是，

安全检查一 指定的初始化式必须确保所有属性 类引入的初始化 超类初始值设定项。

现在，两相初始化从来不讲顺序，但是这个安全检查，引入了super。Init是有序的，在初始化所有属性之后。

安全检查1可能看起来无关紧要， 两阶段初始化可以防止属性值在初始化之前被访问，而不需要进行此安全检查1。

就像这个样本

class Shape {
    var name: String
    var sides : Int
    init(sides:Int, named: String) {
        self.sides = sides
        self.name = named
    }
}

class Triangle: Shape {
    var hypotenuse: Int
    init(hypotenuse:Int) {
        super.init(sides: 3, named: "Triangle") 
        self.hypotenuse = hypotenuse
    }
}

三角形。Init在使用之前已经初始化了所有属性。所以安全检查1似乎无关紧要，

但可能还有另一种情况，有点复杂，

class Shape {
    var name: String
    var sides : Int
    init(sides:Int, named: String) {
        self.sides = sides
        self.name = named
        printShapeDescription()
    }
    func printShapeDescription() {
        print("Shape Name :\(self.name)")
        print("Sides :\(self.sides)")
    }
}

class Triangle: Shape {
    var hypotenuse: Int
    init(hypotenuse:Int) {
        self.hypotenuse = hypotenuse
        super.init(sides: 3, named: "Triangle")
    }

    override func printShapeDescription() {
        super.printShapeDescription()
        print("Hypotenuse :\(self.hypotenuse)")
    }
}

let triangle = Triangle(hypotenuse: 12)

输出:

Shape Name :Triangle
Sides :3
Hypotenuse :12

Here if we had called the super.init before setting the hypotenuse, the super.init call would then have called the printShapeDescription() and since that has been overridden it would first fallback to Triangle class implementation of printShapeDescription(). The printShapeDescription() of Triangle class access the hypotenuse a non optional property that still has not been initialised. And this is not allowed as Two-phase initialization prevents property values from being accessed before they are initialized

因此，确保两阶段初始化是按照定义完成的，需要有一个特定的顺序调用super。init，也就是在初始化self类引入的所有属性之后，因此我们需要一个安全检查1

引用自Swift编程语言，它回答了你的问题:

Swift的编译器会执行四项有用的安全检查来确保这一点 该两阶段初始化完成无错误: 安全检查1 "指定的初始化器必须确保所有的 类引入的属性在它之前初始化 委托到超类初始化式。” 摘自:苹果公司《快速编程语言》。“iBooks。 https://itunes.apple.com/us/book/swift-programming-language/id881256329?mt=11

Swift强制你在使用/可能使用之前初始化每个成员var。因为它不能确定当它变成超级时发生了什么，所以它会出错:小心总比后悔好